Recently, wireless mesh networks attract more and more attention, e.g. for remote control of illumination systems, building automation, monitoring applications, sensor systems and medical applications. In particular, a remote management of outdoor luminaires, so-called telemanagement, becomes increasingly important. On the one hand, this is driven by environmental concerns, since telemanagement systems enable the use of different dimming patterns, for instance as a function of time, weather conditions and season, allowing a more energy-efficient use of the outdoor lighting system. On the other hand, this is also driven by economical reasons, since the increased energy efficiency also reduces operational costs. Moreover, the system can remotely monitor power usage and detect lamp failures, which allows determining the best time for repairing luminaires or replacing lamps.
Current radio-frequency (RF) based wireless solutions use either a star network topology or a mesh network topology. In a star network, a control center has a direct wireless communication path to every node in the network. However, this typically requires a high-power/high-sensitivity base-station-like control center to be placed at a high location (e.g. on top of a building), which makes the solution cumbersome to deploy and expensive. In a mesh network, the plurality of nodes does in general not communicate directly with the control center, but via so-called multi-hop communications. In a multi-hop communication, a data packet is transmitted from a sender node to a destination node via one or more intermediate nodes. Nodes act as routers to transmit data packets from neighboring nodes to nodes that are too far away to reach in a single hop, resulting in a network that can span larger distances. By breaking long distances in a series of shorter hops, signal strength is sustained. Consequently, routing is performed by all nodes of a mesh network, deciding to which neighboring node the data packet is to be sent. Hence, a mesh network is a very robust and stable network with high connectivity and thus high redundancy and reliability.
In the prior art, mesh network transmission techniques can be divided in two groups: flooding-based and routing-based mesh networks. In a flooding-based mesh network, all data packets are forwarded by all nodes in the network. Therefore, a node does not have to make complicated routing decisions, but just broadcasts the data packet. By these means, the technique is quite robust. However, in large networks, the data overhead due to forwarding impacts the overall data rate. Moreover, collisions of data packets are more likely to occur, further reducing the overall performance. Hence, the main problem of this solution is the scalability. Routing-based mesh networks can be further divided into proactive and reactive schemes. In proactive routing-based mesh networks, all needed network paths are stored in routing tables in each node. The routing tables are kept up to date, e.g. by sending regular beacon messages to neighboring nodes to discover efficient routing paths. Although the data transmission is very efficient in such kind of network, the scalability is still low, since in big networks, the proactive update of the routing tables consumes large parts of network resources. Moreover, the routing tables will grow with the scale of the network. In addition, the setup of the network requires time and resources in order to build up the routing tables. Reactive schemes, in contrast, avoid the permanent overhead and large routing tables by discovering routes on demand. They use flooding to discover network paths and cache active routes or nodes. When routes are only used scarcely for single data packets, flooding the data packets instead of performing a route discovery might be more efficient. If routes are kept long enough to avoid frequent routing, reactive schemes degenerate to proactive schemes. An example for a reactive routing-based mesh network protocol is used in ZigBee. However, the main problem of this protocol scheme is still the scalability of the network.
In most sensor/actuator networks, network nodes communicate in general only with a collector node acting as a bridge or gateway to the control center, whereas the collector node (or the control center) is the only entity communicating with individual nodes or a group of nodes. Moreover, communications from the nodes to the control center generally prevail in the overall data traffic. Thus, in these networks, data packet transmission is often optimized from the nodes to the respective collector node, i.e. the underlying protocol is data collector oriented. But these protocols only improve the path towards the collector node. In order to transmit data from the collector node or control center to the individual nodes, either additional extensive routing protocols have to be stored at the single nodes or precise routing information has to be included in the data packets. Though, when using additional protocols, additional storage space for routing tables is required at the nodes, making the nodes and the system more complex. When including precise routing information in the data packet, however, a large data overhead is created, further increasing the network load and thus reducing the network scalability. As an alternative, flooding is often used. Yet, this is a very inefficient way to communicate with an individual node, also resulting in increased network traffic and thus lower scalability.
Moreover, in large-scale multi-hop networks, the number of hops a data packet has to travel is large as compared to a hop distance in small networks. Thus, in a large radio frequency telemanagement system comprising thousands of nodes, 20-40 hops are likely to occur. However, the delivery chance of an individual data packet decreases with its hop distance, since with every hop, there is a chance that the data packet gets lost. Hence, in order to guarantee successful delivery of a data packet, the data packet can be transmitted in acknowledged mode, wherein the receiver node transmits an acknowledgement to the sender node after having received the data packet. Since the receiver node mostly corresponds to the collector node, the acknowledgement has to be transmitted from the collector node to the sender node. However, the corresponding communication paths are very inefficient, as mentioned before, in particular when considering the low payload of an acknowledgement, usually only comprising a type of the data packet and a sequence number.
Hence, a big disadvantage in common wireless mesh networks is constituted on the one hand by the tedious configuration and on the other hand by very limited network scalability. Especially RF telemanagement networks suffer from significant overload due to their topology and size, which limits their scalability. Consequently, efficient routing protocols are required for data transmission from the data collector to individual network nodes or to a group of network nodes in large-scale wireless networks, such as street illumination systems with a high number of luminaire nodes, in order to achieve the required throughput, response times and robustness.
US 2009/0154395 A1 describes routing in a wireless sensor network having a hierarchical structure, wherein the network comprises a plurality of clusters, each with a plurality of nodes and a cluster head. The cluster head functions as a gateway between the nodes of the corresponding cluster to nodes of other clusters.